Prosecute to restrict member that's lying (6)
I believe the answer is:
Here is my best explanation:
'lying' is the definition.
'prosecute to restrict member' is the subsidiary indication.
'prosecute' becomes 'sue' (I've seen this before).
'to restrict' means one lot of letters goes inside another.
'member' becomes 'pin' (pin is a kind of member).
'sue' enclosing 'pin' is 'supine'.
'that's' is the link.
(Other definitions for supine that I've seen before include "Inert; on one's back", "On one's back", "offering no resistance", "Passive", "Prostrate", "Unresisting", "Slow in acting", "Lying face upwards, indolent", "flat out" and "looking up".)